As one example of the compound of this kind, there is known an exhaust purifying catalyst, in which fine particles having a NOx occluding function or fine particles of a precious metal or the like having an oxidizing catalyst function. A method for producing that exhaust purifying catalyst by carrying the fine particles on a porous carrier is described in Japanese Patent Laid-Open No. 05-261287. This method can be called the water-adsorption carriage method. In this method, a solution, which is prepared by dissolving a salt of a metal such as an alkali metal or an alkali earth metal to become carried fine particles is dissolved in water, is absorbed by a carrier and is dried. The salt, which has deposited as the water evaporates, is sintered into the oxide and is fixed on the carrier.
As this carrier, there has been usually employed a porous substance having a number of pores for increasing its surface area. The aforementioned solution having the aforementioned salt dissolved is absorbed to penetrate by the porous substance so that the salt is widely dispersed. In this case, the drying phenomena due to the water evaporation occur on the surface of the carrier, and the capillary pressure to occur at the open end portions of the pores is high. Therefore, the solution having the salt dissolved therein is moved, while being dried, to the surface side by the capillary pressure. As a result, the salt concentration rises on the surface side of the carrier, and the water evaporates in this state. Therefore, the particle diameter of the salt having deposited becomes larger on the surface side of the carrier.
In case the exhaust gas purifying catalyst containing the NOx occluding material is produced, for example, the particle size of the NOx occluding material is enlarged if the aforementioned water-adsorption carriage method is adopted using the NOx occluding material as the carried fine particles. As a result, the sulfur poisoning (i.e., S poisoning) of the NOx occluding material in the catalyst obtained is not only easy to proceed but also hardly recoverable. In the exhaust gas purifying catalyst of this kind, moreover, the concentration of the catalytically active substance such as a precious metal is set higher on the surface side of the coating layer and lower on the lower layer side. During the carrying procedure of the NOx occluding material, however, the concentration of the NOx occluding material becomes gradually higher on the surface side of the carrier. As a result, the catalytically active substance such as the precious metal is covered with the NOx occluding material, and it is highly possible that the degree of exposure of the catalytically active substance such as the precious metal becomes lower.
Here in the water-absorption carriage method, the carriage is partially dissolved in the procedure while the salt of the aforementioned NOx occluding material is being dried. As a result, it is also possible that the heat-resistance of the carrier drops. Thus in the water-absorption carriage method of the prior art, the particle diameter of the carried fine particles easily become larger, or the heat-resistance of the carrier easily drops. In case the exhaust gas purifying catalyst containing the NOx occluding material is to be produced, it is also possible that the NOx purifying performance or the oxidation/reduction performance as the catalyst drops.
On the other hand, it is also possible to prepare the carried fine particles in a solid without employing the solution having the metal salt of the carried fine particles dissolved therein. In this case, the carried fine particles are protected with a stabilizing agent of a high polymer or a surface active agent so that the carried fine particles may be suppressed in their mutual agglomeration. As a result, the carried fine particles become expensive. In the presence of the stabilizing agent, moreover, the carried fine particles are hard to carry on the carriage. Even with the use of the stabilizing agent, moreover, the carried fine particles are hard to keep the stable state for a long time, and their management may be troublesome.